Algae are an extremely diverse group of aquatic plants. They range in size from single cells around a micrometre (1/1000 millimetre) in diameter to seaweeds the size of trees. Like other plants, they use the energy of sunlight to convert carbon dioxide and water into sugars and oxygen. Algae occur widely in Antarctica.

By far the most important are the marine phytoplankton—microscopic floating single cells. The 350 or so different species identified from Antarctic waters exhibit a huge diversity of size, shape, lifestyle, and food value to grazers. In addition to being the base of the food web on which essentially all other marine organisms depend, phytoplankton play a significant role in influencing global climate.

The Southern Ocean absorbs atmospheric carbon dioxide, the principal greenhouse gas. This is largely due to the uptake of carbon dioxide by phytoplankton. In addition, some produce sulphur-containing compounds which when released to the atmosphere form aerosol particles that promote the formation of clouds. Clouds are important in climate control because they reflect much of the Sun’s energy back into space. Thus phytoplankton can influence regional climate.

As well as the phytoplankton, the sea ice around Antarctica often has rich growths of algae, often on the underside. Some 700 seaweeds have been reported from the Southern Ocean of which some 35% are found no where else. In addition to seaweeds, the sea floor in shallow waters is covered with single-celled algae which provide food for bottom-dwelling animals.

On the Antarctic continent and subantarctic islands, algae live in lakes and streams, on moist soil and in snow banks. On the Antarctic continent, some algae also live in the spaces between the grains of porous sandstone rocks and underneath translucent quartz rocks where moisture and light are available for their growth. Soil algae, together with their associated bacteria, are ecologically important as they contribute to the organic material and help bind the soil particles together with the mucilage and slime they secrete.

There are currently some 700 recorded species of terrestrial and aquatic algae in Antarctica. They have been collected from as far south as 86° 29'.

Microscopic algae are also found in snow and ice on land in the coastal regions. In summer the algae accumulate in sufficient numbers to colour the snow banks red, green, orange and even grey. Snow algae are generally single-celled organisms, although some multicellular and filamentous forms exist.

Often overlooked are algae that constitute the photosynthetic part of lichens. Even though there are only a few species of lichen algae, they are nevertheless important as there is a significant lichen flora in Antarctica.

The species composition and abundance of Antarctic algae are controlled by many environmental variables. Wind-driven mixing and iron availability are major controls in the sea while nutrients and availability of water are important in non-marine habitats. Increased UVB radiation, resulting from the springtime depletion of stratospheric ozone (the ozone hole), has very different effects on different Antarctic algae. Algae use several mechanisms to protect themselves from UV exposure. These include avoidance, protective pigments and repair mechanisms. Increasing UVB is likely to lead to a change in the composition of algal communities with the more UV tolerant species dominating.